Sprinklers are widely used in fire suppression applications. The function of the sprinkler is to prevent fire growth by effectively dispersing water (or other liquid, or foam) over a wide area within the fire environment area. This dispersion is achieved by breaking a continuous stream of liquid into a spray of discrete drops with wide range of sizes and velocities.
While fire suppression sprays control the fire through a number of mechanisms which include wetting, cooling, blowing, oxygen depletion, and radiation attenuation, the primary suppression mechanism provided by sprinklers is wetting.
The suppression performance of the sprays produced by sprinklers is determined by their ability to penetrate the fire to reach burning surfaces, while dispersing water (or other appropriate liquid) throughout the heated fire environment area. Spray penetration and dispersion are governed by the initial spray characteristics (initial drop size, and velocity characteristics of the spray) and their interaction with the fire.
Sprays with large drops readily penetrate the fire plume to wet combustible materials and control fire growth. However, these sprays require a high volumetric flow rate for effective dispersion.
Alternatively, sprays with small drops effectively use their volume to generate a large number of drops facilitating dispersion. These small drops also have a high surface to volume ratio for rapid evaporation, reducing fire gas temperatures and associated heat feedback to fuel surfaces. However, small drops easily lose their initial momentum making it difficult to penetrate the fire plume.
Optimizing the drop size for fire plume penetration and dispersion is critical for fire suppression performance.
Traditionally, sprinkler performance has been evaluated through testing. However, with the advent of the Fire Dynamics Simulator (FDS) first released in 2000 [K. McGrattan, et al., Fire Dynamics Simulator (Version 5), Technical Reference Guide, NIST Special Publication 1018-5, 2008], modeling of fire phenomena with computational fluid dynamics (CFD) tools is becoming increasingly popular.
Some early computational studies [R. L. Alpert, Fire Safety J., 9 (1), 1985, 157-163; W. K. Chow, et al., Fire Safety, J., 17, 1991, 263-290; S. Nam, Fire Safety J., 26 (1), 1996, 1-34; S. Nam, Fire Safety J., 32 (4), 1999, 307-330; S. Nam, Atomization and Sprays, 4, 1994, 385-404; and L. A. Jackman, “Sprinkler Spray Interaction with Fire Gases,” PhD thesis, South Bank University, London, UK, 1992] are focused on studying the interaction between fire plumes and sprinkler sprays. However, these early computational studies did not provide detailed knowledge of initial spray characteristics, dispersion predictions (typically quantified through analysis of volume flux to the floor), and they were not sufficient in producing accurate and quantifiable results as to sprinkler performance.
Early spray characterization focused on far-field measurements are presented in P. H. Dundas, “The Scaling of Sprinkler Discharge: Prediction of Drop Size,” Report No. 10, Factory Mutual Research Corporation, 1974; G. Heskestad, “Proposal for Studying Interaction of Water Sprays with Plume in Sprinkler Optimization Program,” Factory Mutual Research Corporation, 1972; and H. Z. Yu, “Investigation of Spray Patterns of Selected Sprinklers with the FMRC Drop Size Measuring System,” First International Symposium on Fire Safety Science, New York, p. 1165-1176.
Most of these studies are focused on volume flux distribution and drop size measurements. Volume flux distribution is a major criterion for sprinkler evaluation, since it shows the ability of a sprinkler to effectively disperse water over the protected area. Volume flux usually has a very high peak directly below the sprinkler, and decreases dramatically when moving radially outwards. Despite this high peak, only a small portion of the overall flow is contained in this centerline area making it relatively unimportant to sprinkler performance.
Drop size characterization measurements primarily focused on quantifying the volume median diameter, dv50, were obtained from drop size distributions in sprinkler sprays. P. H. Dundas in [“The Scaling of Sprinkler Discharge: Prediction of Drop Size,” Report No. 10, Factory Mutual Research Corporation, 1974] used a high-speed photographic and laser shadowing techniques to measure drop size distributions from six sprinklers with nozzle diameters ranging from 3.1-25.4 mm and with pressures ranging from 0.345-5.25 bar.
Dundas's research confirmed the correlation first proposed by G. Heskestad in [“Proposal for Studying Interaction of Water Sprays with Plume in Sprinkler Optimization Program,” Factory Mutual Research Corporation, 1972] thatdv50/D0=CWe−1/3,  (Eq. 1)
where D0 is the orifice diameter, C is a constant depending on the sprinkler geometry, and We is Weber Number defined as We=ρU2/σ,
where ρ is the density of the fluid, U is the velocity of the fluid, and σ is the surface tension.
Dundas summarized the C value from different researchers showing values in the range 1.74<C<3.21.
Detailed sprinkler measurements have also been reported by H. Z. Yu in [“Investigation of Spray Patterns of Selected Sprinklers with the FMRC Drop Size Measuring System,” First International Symposium on Fire Safety Science, New York, pp. 1165-1176], J. M. Prahl, et al., in [Fire Safety J. 14 (1988) 101-111], J. F. Widmann in [Fire Safety J. 36 (2001) 545-567], and D. T. Sheppard in [Spray Characteristic of Fire Sprinkler, NIST GCR 02-838, 2002]. They also verified that drop size could be reasonably correlated with We−1/3. However, the respective coefficients vary with sprinkler configuration.
The overall measured drop size was used to generate sprinkler sprays in early spray dispersion modeling studies. Notable work on sprinkler spray modeling has been conducted by R. L. Alpert and presented in [R. L. Alpert, Fire Safety J., 9 (1) (1985) 157-163]. Further improvements were performed by Bill [R. G. Bill, Fire Safety J. 20 (1993) 227-240] and Nam [S. Nam, Fire Safety J., 26 (1) (1996) 1-34]. In their study, the sprinkler spray was introduced by assigning the measured drop size, volume flow rate, discharge speed and discharge angle of 275 trajectories. The trajectories were adjusted manually so that the predicted volume density on the floor would match the experiments.
Similar ideas have been incorporated into CFD (Computational Fluid Dynamics) tools where the user can map out the initial spray by specifying the local velocity and flux fraction details for arbitrary solid angles. However, tabulating these values for the entire sprinkler spray is computationally prohibitive. Furthermore, the ability to include local drop size information at a given solid angle is required to completely characterize the spray.
A comprehensive methodology for characterizing sprinkler sprays in effective and accurate fashion, free of shortcoming of the early studies is a long-lasting need for fire suppression systems.